The use of current mode switching regulators as a means of providing a predetermined and substantially constant output voltage to a variable load from a fluctuating voltage source is well known. FIG. 1 illustrates an example of the use of a high-side switch 12 in a trailing edge modulation current mode switching regulator. As shown in FIG. 1, the circuit 1 includes a controller 10 having a first output coupled to the high-side switch 12 and a second output coupled to a low-side switch 14. The circuit further includes inductor L having one end coupled to the drains of the high-side switch 12 and low-side switch 14, and the other end coupled to a load capacitor C, where the voltage across the capacitor represents VOUT (i.e., the voltage supplied to the load). The output voltage, VOUT, is also fed-back and compared to a reference voltage, VREF, with the difference between the two being fed to a second input of the controller 10.
The circuit illustrated in FIG. 1 can be utilized, for example, in essentially any “trailing edge modulation” switching regulator that terminates the high side switch “on” interval based on inductor current, and initiates the high-side switch “on” interval by a clock for fixed frequency operation, or by a timer for predetermined “off” time operation.
FIG. 2 illustrates, in detail, a prior art version of the high-side switch 12 of FIG. 1 in a circuit which provides for sensing of the current IL flowing through the inductor L. As shown in FIG. 2, the high-side switch 12 comprises a PMOS transistor 12a and an inherent body diode 12b. The source and drain terminals of the high-side switch 12 are coupled to the inputs of a sense amplifier 24, which operates to generate an output representing the current flowing through the high-side switch 12. The output of the sense amplifier 24 is represented as ISENSE=ISWITCH×RON×Gm, where ISWITCH corresponds to the current flowing through the high-side switch 12, RON corresponds to the “ON” resistance of switch 12 and Gm corresponds to the transconductance of amplifier 24. The remaining components of the circuit are the same as shown in FIG. 1.
In operation, the circuit illustrated in FIG. 2 functions to sense the voltage drop across the high-side switch 12 when it is “ON” and generate the current signal, ISENSE. The circuit of FIG. 2 works reasonably well with MOS switches having an approximately linear “ON” resistance regardless of the magnitude or direction of the switch current, ISWITCH. However, the “ON” resistance of the switch has significant variations from unit to unit as well as with temperature, etc., which can result in unacceptable variations in performance. In order to overcome these variations, it has been known to utilize a matching or replica device having a scaled but tracking value of RON as shown in FIG. 3.
Referring to FIG. 3, the circuit includes the high-side switch 12, inductor L and load capacitor C coupled in the same manner as shown in FIG. 2 above. The circuit further includes a replica switch 32, a sense amplifier 34 and an auxiliary device 36, which are coupled within the circuit as follows. The replica switch 32, which in the given embodiment is a PMOS switch, is configured so as to have the same drive signals as the high-side switch 12. As shown, the source terminal of both the replica device 32 and the high-side switch 12 are coupled to the supply voltage 31, and the gate terminal of both the replica device 32 and the high-side switch 12 receive the same input signal. The drain terminal of the high-side switch 12 is coupled to the non-inverting terminal of the sense amplifier 34 and the drain terminal of the replica device 32 is coupled to the inverting terminal of the sense amplifier 34. The drain terminal of the replica device 32 is also coupled to the source terminal of the auxiliary device 36, which in the given embodiment is a PMOS switch. The gate of auxiliary device 36 receives the output of the sense amplifier 34 as an input signal, and the source terminal of switch 36 generates an output signal ISENSE, which is equal to K*ISWITCH, where K is equal to the scaling factor of the replica device 32.
By using a replica device 32 which exhibits a reduced area relative to the high-side switch 12 (i.e., scaling the replica device), it is possible to substantially negate the variations in RON. As shown in FIG. 3, the value of RON is increased by a factor of K, which is the scaling factor. As an example, a scaling factor on the order of 1/1000 may be utilized. In operation, utilizing the foregoing feedback configuration, the sense amplifier 34 operates to maintain the voltage across the replica device 32 equal to the voltage across the high-side switch 12. When ISWITCH is positive, the sense amplifier 34 functions to turn on the auxiliary device 36, and the auxiliary device outputs the ISENSE signal. Thus, when ISWITCH is positive (i.e., current flowing from VSUPPLY to the load), ISENSE, which is a scaled version of ISWITCH, is flowing through the replica device 32 and the auxiliary device 36. It is noted that the ISENSE signal is typically coupled to the controller 10 which governs the overall operation of the switching regulator.
However, in the event that the current flowing through the high-side switch 12 becomes negative (i.e., current flowing from the load back to VSUPPLY), the drain of the high-side switch 12 becomes positive relative to the source of the high-side switch 12, which causes the non-inverting terminal of the sense amplifier 34 to be positive relative to the inverting terminal of the sense amplifier 34. As a result, the sense amplifier 34 generates an output signal which causes the auxiliary device 36 to turn off, thereby turning off the ISENSE signal.
Thus, while the circuit of FIG. 3 is acceptable for use when the current flowing through the high-side switch is positive, it is not capable of handling both positive and negative current flow through the high-side switch. As such, the circuit of FIG. 3 cannot be utilized, for example, in a switching regulator operating in a synchronously rectified current mode where ISWITCH is negative because the low side switch has been kept “ON” in order to lower the output voltage with current “negative I”, which recirculates as negative ISWITCH when the low side switch turns OFF.
Accordingly, there exists a need for both a method and an apparatus which allows for the negation of the effects of variations and fluctuations in RON in the high-side switch and which allows for substantially loss-less sensing of the current flowing though the high-side switch in both a positive and negative direction.